"I had to take the shape of someone you know in order to appear, because I'm actually quite weak. I can't do anything for you Trisha, sorry." (-the Subaudible's manifestation speaking to Trisha)
"That's because you got used to it, but take my word, Trish, that sound was always there. Even in a house where there aren't any baseboard heaters, there are noises. The fridges goes on and off. The pipes thunk. The floors creak. The traffic goes by outside. We hear those things all the time, so most of the time we don't hear them at all...I don't want to believe in a God who would deliberately create bad people and then deliberately send them to roast in a hell He created--but I believe there has to be something"
(-Trisha's father explaining the Subaudible)
The Subaudible is the religion made up by the otherwise aetheistic father of Trisha MacFarland , Larry MacFarland. It is called this name because Larry describes it as something unnoticable but significant and everpresent, like the buzzing of a radiator or the creaking of floorboards.
Trisha constantly thinks about the Subaudible while lost in the woods, most likely because she so idolizes her father. When it manifests itself and speaks to her, alongside the God of Tom Gordon and the God of the Lost, it looks like Larry and wears a black, hooded cloak that completely shadows its face. It explains that it is too weak to assume a unique form, and so it has to look like her father in order to address her.
The Subaudible as a manifestation is described as apologetic and sheepish, most likely alluding to the real subaudible being insignificant and quiet. Trisha notices that it is drunk, saying that she can smell the alcohol from across the stream (alcoholism being something Trisha deeply dislikes about her father). It does not deny this.
Other than the manifestation/person, the Subaudible has no physical features or personality.